As methods for measuring the porosity of a sample of particle powder, there are known a gas adsorption method and a mercury injection method. In these methods, the surface area of the sample in a dried state is evaluated, and then, the average porosity of the sample is calculated according to the relationship with the average grain diameter or the mass (see Patent Literatures 1 and 2, for example). A method of measuring an average porosity using NMR is also known.